This invention relates to a process for the treatment of waste gas discharged from a cyclohexane oxidation reactor for the manufacture of adipic acid that allows the recovery of valuable materials contained in the waste gas.
During the one-step oxidation of cyclohexane to adipic acid (for example, based on the processes disclosed in U.S. Pat. No. 5,321,157 and international patent application WO 94/07834), gaseous, supercritical, and thus non-condensable components are discharged from the reactor. It is necessary to deal on one hand with the gaseous reaction products carbon monoxide (CO) and carbon dioxide (CO.sub.2), as well as on the other hand with the feed materials O.sub.2 (added in small excess amounts) and nitrogen gas N.sub.2 (which is included when, preferably, air or oxygen-enriched air is used as the oxidation agent). Nitrogen normally forms the main component of the waste gas (80 to 90% by weight during oxidations with air). Together with these supercritical, inert components, other materials corresponding to the thermodynamic equivalent weight composition --for example, the cyclohexane feed material, the acetic acid solvent, the intermediate product cyclohexanone, and the water produced as a reaction product --are discharged in the vapor phase. These other materials can be only partially separated by condensation, with the solidification point of the acetic acid (about 15.degree.C.) acting as a primary limiting factor. The waste gas therefore still contains a high percentage of valuable products, especially cyclohexane and acetic acid, which should be recovered as completely as possible for cost and environmental reasons. This is especially important when using small cyclohexane throughputs. In addition, the waste gas must be cleaned of carbon monoxide prior to discharge into the atmosphere.
The inventive process described below provides the solution to this task, and has the following main advantages:
(i) the materials used as the absorption solution are materials already present in the system, which means that no contamination of the process by foreign material can occur during the recovery of the valuable materials cyclohexane and acetic acid, PA1 (ii) the absorption liquids can be made available in required quantities from the process itself, without significant additional preparation costs, and the absorbates can be recycled to the process at appropriate points, such that no additional expenditures occur for the preparation of the materials required and the removal of residual materials. PA1 (a) scrubbing the waste gas from the cyclohexane oxidation reactor with acetic acid in a first absorption step (optionally after cooling the waste gas to reduce the amount of condensable vapors) to reduce the amount of cyclohexane to a residual content of less than about 1% by weight, PA1 (b) scrubbing the waste gas discharged from the first absorption step with water in a second absorption step to reduce the amount of acetic acid to a residual content of less than about 1% by weight, and PA1 (c) subjecting the waste gas discharged from the second absorption step to an oxidation step to oxidize carbon monoxide and other oxidizable components of the waste gas to carbon dioxide and water.